Process of manufacturing a sieve plate having apertures of nonuniform crosssection



E M LT AM 2 TP Em H N CH RAW N6 0 NFH E O HW E TE w MCR AU H O UR CNE AP MA JFG m 3 Aw m Dec. 19, 1967 Filed March 12, 1965 HANS'JOACH/M HEINRICH L ER Hm mm m m a H 1 Attorney 1967 HANS'JOACHIM HEINRICH ETAL 3359'192 PROCESS OF MANUFACTURING A SIEVE PLATE HAVING APERTURES OF NONUNIFQRM Filed March 12, 1965 CROSS'SECTION 2 Sheets-Sheet 2 FIG. 7

HANS 'JOACH/M HEINRICH M41. T [R OIETZEL INVENTOR.

A Horn 19] United States Patent 3 359 192 Paocnss on MANUFACTURING A SIEVE PLATE HAVING APERTURES or NONUNIFORM cnoss- Claims. (Cl. 204143) Our present invention relates to sieve plates and to a process for making same.

It is known to manufacture sieve plates from foils consisting of various metals and alloys, such as stainless steel, by covering the surfaces of these foils with masking layers having a multiplicity of perforations through which these foils can be locally eroded, by chemical etching or electrolytic treatment, whereby transverse passages are formed in the foils at the location of the perforations. Suitable masking layers can be applied 'by printing or offset techniques; a typical method includes the coating of the foil with a photosensitive emulsion, the perforations being produced in that case jection of the desired sieve pattern onto the emulsion and removal of unexposed portions of silver halide by successive developing and fixing. If the foil so masked is eroded from one surface only, the resulting holes are more or less frustoconical and converge toward the opposite surface; thus even minor variations in the thickness of the foil may greatly change the diameter of the holes at the opposite surface, i.e. the minimum diameter which determines the mesh size of the sieve. S-uch onesided erosion, in general, is satisfactory only for foil thicknesses of less than 0.1 mm.; foils up to twice that thickness may be perforated in this manner by simultaneous erosion from opposite surfaces whereby doubly frustoconical holes converging at the center are formed; here, too, the minimum hole diameter varies with the thickness of the foil.

The general object of our present invention is to provide a process for making sieve plates of this character in which the critical minimum hole diameter is substantially independent of foil thickness and is, therefore, virtually unaffected by variations in that thickness.

Another object of this invention is to provide a sieve plate, produced from metallic foil by a process of the general type described above, whose thickness may be greater than 0.2 mm.

It is also an object of our present invention to provide a process of making such sieve plates in a manner designed to reduce the conicity of the holes to a minimum whereby the number of holes per unit area can be increased.

In practicing our invention, we provide each side of a metallic foil (e.g. of stainless steel) with a perforated masking layer in the well-known manner and thereupon subject the foil to an initial erosion step through the perforations of at least one layer to a depth which is insufficient to penetrate the foil (thus, to a depth equal to a minor fraction of the foil thickness if erosion proceeds simultaneously from both sides), this being followed by -a packing of the resulting erosions on one side (i.e. the working side of the sieve) with a protective agent and by an erosion or further erosion of the opposite or reverse side of the foil to the level of the erosion on the working side, thus until a breakthrough is achieved. In view of the limited depth of the initial erosion, the recesses first formed at the working side depart only very slightly from a cylindrical shape so that the diameter of each recess is almost the same at different cross-sections; this diameter, in practice, will be only a little larger than by photographic pro- 3,359,192 Patented Dec. 19, 1967 2 that of the perforations in the masking layer through which the recess was formed.

Upon the subsequent removal of the protective agent and the masking layers, the holes so obtained have a minimum width corresponding to that of the initial recesses on the working side, thus a width which is almost independent of the depth to which the reverse side must be eroded to achieve the breakthrough. Variations in foil thickness will effect only the extent to which the reverse side must be eroded in order that the recesses on both sides will merge; local deviations in this thickness will therefore merely result in slight variations of the distance of the point of breakthrough, i.e. the location of the narrowest cross-section, from the working surface, yet this will not materially affect the hole diameter at that crosssection since this diameter will correspond to that of the shallow recess on the working side which has only negligible conicity.

If the initial erosion is carried out by chemical means alone, the subsequent erosion of the reverse side of the foil may be performed either by etching or electrolytically; the latter type of treatment produces more slender cones, in view of the directive action of the applied electric field. A reduction in conicity is also possible by the addition of a conventional passivating agent (e.g. sodium silicate or a phosphide) to the bath to protect the flanks of the recess from electrolytic or chemical attack.

We have found that our improved process can be used in the manufacture of sieve plates well over 0.2 mm. in thickness. In such cases it will be generally desirable to limit the depth of initial erosion on the working side to less than 0.1 mm. so as to leave a thickness of more than 0.1 mm. to be penetrated from the reverse side.

Sutiable protective agents, available in paste form, include asphalt emulsions which can be readily removed upon drying.

The invention will be described in greater detail hereinafter, reference being made to the accompanying drawing in which:

FIG. 1 is a fragmentary sectional view (on a greatly enlarged scale) of a metal foil to be transformed into a sieve plate according to our invention;

FIG. 2 is a view similar to FIG. 1, illustrating the coating of the foil with perforated masking layers;

FIGS. 36 are views similar to FIGS. 1 and 2, illustrating the progressive erosion of the foil to produce a breakthrough; and

FIG. 7 is an enlarged cross-sectional view of part of a completed sieve plate, illustrating the effects of variations in foil thickness upon the hole size.

In the practice of our process, a metal foil 10 (FIG. 1) is first coated on its two sides with a pair of masking layers 11', 11" (FIG. 2) each having a multiplicity of perforations 12, 12", respectively. Layer 11' coats the working side of what is to become the sieve plate, i.e., the side from which a fluid to be filtered or screened is fed through the plate, the layer 11" thus overlying the reverse side of the plate. Next, as illustrated in FIG. 3, the foil 10 is attacked by an electrolyte or an etching bath so that recesses 13 are formed through the perforations 12' on the working side of the foil. If the plate is com pletely immersed in the bath, similar recesses 13" of substantially the same depth will be formed simultaneously through the perforations 12" of layer 11" unless the reverse side of the plate is protected against erosion. As the formation of the recesses 13" is ordinarily not objectionable, such protection will generally not be required.

The erosion of the foil to form the recesses 13, 13 (or at least the former) is halted at a certain depth of penetration D, generally less than half the overall thickness T of the foil. At this point each of the recesses, and

15 particularly the recesses 13, will converge only slightly to a level L whose distance d from the working surface of the foil is somewhat less than depth D and represents a minor fraction of thickness T. Down to the level L, therefore, the recesses 13 are practically cylindrical in shape.

FIG. 4 illustrates the filling of the recesses 13' by a protective paste 14 which prevents further erosion. Thereafter, as illustrated in FIG. 5, foil is subjected to continuing erosion with progressive widening and deepening of the recesses 13" and undercutting of the foil underneath layer 11. In the terminal stage of the erosion process, illustrated in FIG. 6, the reverse-side recesses. 13 have reached the level L so as to merge with the corresponding working-side recesses 13 to form a throughgoing passage 13 whose diameter at its narrowest point, i.e., in the vicinity of level L, is substantially equal to the diameter of the perforations 12'. Finally, the layers 11' and 11" are ground off or otherwise removed, the filler 14 is extracted and the resulting sieve plate 20, shown partly in FIG. 7, is ready for use.

Although normally the two surfaces 21', 21 of the sieve plate 2 may be considered parallel, we have illustrated an exaggerated deformation of reverse surface 21" to illustrate the effect of variations in foil thickness upon the shape of the resultant passages 13a, 13b, 130. It will be seen that, in view of the almostinsignificant coincity of the recesses 13 (FIGS. 36), the minimum width w of the passages is practically the same even as the plane of breakthrough, designated P in FIG. 7, approaches or recedes from working surface 21 with increasing or decreasing foil thickness. Thus, thickness T is the sum of the distance d from surface 21 to plane P and the distance d" from that plane to the surface 21"; since distance d" depends on the duration of the final erosion step and is constant for the entire foil, the depth d of the waist portion of passages 13a-13c varies in accordance with Td" so that the effective depth did of initial penetration d should always exceed the maximum variation in foil thickness. When this requirement is satisfied, the object of producing a sieve plate with effective hole sizes substantially independent of foil thickness is realized.

The conicity or degree of convergence of the recesses 13" can be controlled with the aid of the aforementioned passivating agents and, under otherwise equal conditions, will be less for an electrolytic treatment than for a purely chemical etching process. Best results are obtained with foils upwards of about 0.2 mm. in thickness if d is somewhat less than 0.1 mm. and T-d is substantially greater than 0.1 mm.

In the case of electrolytic erosion it is desirable to carry out repeated current reversals and/or agitation of the electrolyte in order to help removing polarization or impurities, thereby making the surface of the recess more uniform.

We claim:

1. A process for making a sieve plate with a multiplicity of orifices, comprising the steps of covering one surface of a metallic foil with a first masking layer having a multiplicity of perforations, covering the opposite side of said foil with a second masking layer having a multiplicity of perforations aligned with those of said first layer, eroding said one surface through the perforations of said first layer to a limited depth less than the thickness of said foil, packing the resulting erosions with a protective agent, terminating the erosion of said one surface through said first layer prior to throughgoing passage, eroding said opposite side through the perforations of said second layer to the level of the erosions of said one side whereby throughgoing passages converging from said opposite side to said one side are formed in said plate, and removing said masking layers and said agent.

2. A process as defined in claim 1 wherein said foil has a thickness of 0.2 mm., said limited depth being less than 0.1 mm.

3. A process as defined in claim 1 wherein the erosion of said opposite side is carried out in the presence of a passiv-ating agent for the flanks of the erosions.

4. A process as defined in claim 1 wherein the erosion of both said sides is performed by etching.

5. A process as defined in claim 1 wherein the erosion of said opposite side is performed electrolytically.

6. A process as defined in claim 5 wherein electrolysis is carried out intermittently with intervening polarity reversals.

7. A process as defined in claim 5 wherein electrolysis is carried out in a treatment bath with agitation thereof.

8. A process for making a sieve plate with a multiplicity of generally frustoconical orifices having narrow ends of predetermined width all facing in the same direction, comprising the steps of covering one surface of a metallic foil with a first masking layer having a multiplicity of perforations corresponding in size to said narrow ends, covering the opposite side of said foil with a second masking layer having a multiplicity of perforations aligned with those of said first layer, eroding said one surface through the perforations of said first layer to a limited depth, less than the thickness of said foil, packing the resulting erosions with a protective agent, terminating the erosion of said one surface through said first layer prior to throughgoing passage, eroding said opposite side through the perforations of said second layer to the level of the erosions of said one side whereby throughgoing passages converging from said opposite side to said one side are formed in said plate, and removing said masking layers and said agent.

9. A process for making a sieve plate with a multi plicity of orifices, comprising the steps of covering one surface of a metallic foil with a first masking layer having a multiplicity of perforations, covering the opposite side of said foil with a second masking layer having a multiplicity of perforations aligned with those of said first layer, eroding said one surface through the perforations of said first layer to a limited depth substantially less than half the thickness of said foil, packing the resulting erosions with a protective agent, eroding said opposite side through the perforations of said second layer to the level of the erosions of said one side whereby throughgoing passages converging from said opposite side to said one side are formed in said plate, and removing said masking layers and said agent.

10. A process for making a sieve plate with a multiplicity of generally frustoconioal orifices having narrow ends of predetermined width all facing in the same direction, comprising the steps of covering one surface of a metallic foil with a first masking layer having a multiplicity of perforations corresponding in size to said narrow ends, covering the opposite side of said foil with a second masking layer having a multiplicity of perforations aligned with those of said first layer, eroding said foil through the perforations of each layer to a depth substantially less than half the thickness of said foil, packing the resulting erosions on said one surface with a protective agent, further eroding said opposite side through the perforations of said second layer to the level of the erosions of said one side whereby throughgoing passages converging from said opposite side to said one side are formed in said plate, and removing said masking layers and said agent.

References Cited UNITED STATES PATENTS 2,469,689 5/1949 Gresham 204-443 2,663,821 12/1953 Law 156-7 3,179,543 4/1965 Marcelis 204-143 FOREIGN PATENTS 1,340,843 9/ 1963 France.

ROBERT K. MIHALEK, Primary Examiner. 

1. A PROCESS FOR MAKING A SIEVE PLATE WITH A MULTIPLICITY OF ORIFICES, COMPRISING THE STEPS OF COVERING ONE SURFACE OF A METALLIC FOIL WITH A FIRST MASKING LAYER HAVING A MULTIPLICITY OF PERFORATIONS, COVERING ONE SURFACE OF A METALLIC FOIL WITH A FIRST MASKING LAYER HAVING A MULTIPLICITY OF PERFORATIONS, COVERING THE OPPOSITE SIDE OF SAID FOIL WITH A SECOND MASKING LAYER HAVING A MULTIPLICITY OF PERFORATIONS ALIGNED WITH THOSE OF SAID FIRST LAYER, ERODING SAID ONE SURFACE THROUGH THE PERFORATIONS OF SAID FIRST LAYER TO A LIMITED DEPTH LESS THAN THE THICKNESS OF SAID FOIL, PACKING THE RESULTING EROSIONS WITH A PROTECTIVE AGENT, TERMINATING THE EROSION OF SAID ONE SURFACE THROUGH SAID FIRST LAYER PRIOR TO THROUGHGOING PASSAGE, ERODING SAID OPPOSITE SIDE THROUGH THE PERFORATIONS OF SAID SECOND LAYER TO THE LEVEL OF THE EROSIONS OF SAID ONE SIDE WHEREBY THROUGHGOING PASSAGES CONVERGING
 5. A PROCESS AS DEFINED IN CLAIM 1 WHEREIN THE EROSION OF SAID OPPOSITE SIDE IS PERFORMED ELECTROLYTICALLY. 